1. Field of the Invention
The present invention relates to a load controller that controls the load in the equi-power mode.
2. Related Prior Arts
An U.S. Pat. No. 7,812,594, has disclosed an optical device and a method to control the device. The optical device disclosed therein provides an optical waveguide whose refractive index may be adjusted by a heater integrated within the device. Generally, the heater may be controlled in the equi-power mode by monitoring at least one of a current flowing therein and a voltage applied to the heater.
The optical device reported in a conference of OPE 2008-29 (LQE2008-30) held in June, 2008, is a type of a tunable laser diode (hereafter denoted as LD) that includes an semiconductor optical amplifier (SOA), a distributed feedback LD (DFB-LD), and a chirped sampled grating distributed Bragg reflector (CSG-DBR), where those elements are integrated within a common semiconductor substrate. The reflection spectrum of the CSG-DBR may be adjusted by the heater, specifically, an average temperature and temperature distribution thereof, also the emission spectrum of the DFB-LD may be adjusted by an average temperature of the DFB-LD.
When the heater is control in the equi-power mode to control the output wavelength of the tunable LD, a precise monitoring of the current and the voltage applied to the heater are sometimes necessary. In particular, when the tunable LD is applied to the DWDM (Dense Wavelength Division Multiplexing) system, the power provided to the heater is necessary to be precisely kept constant.
For the equi-power control of the heater, voltages in both ends of the heater and a current flowing therein are monitored; and the voltage and the current are controlled such that the electrical power calculated from the monitored voltages and the current becomes equal or close to the target power. FIG. 5 is a block diagram of a conventional heater controller 100 that monitors both of the current and the voltage. The heater controller 100 provides a current source 101 to provide current for the heater H, a current monitor 102 to monitor the current, a controller 103 to control the current. The controller 103 receives the current data Imon from the current monitor 102 through the analog-to-digital converter (hereafter denoted as A/D-C) 104, and two voltage data, VHmon and VLmon, which correspond to voltages measured at both ends of the heater H through the A/D-C 104. The controller 103 calculates the electrical power at the heater H from those data, and generates a control signal to the current source 101 through the digital-to-analog converter (hereafter D/A-C) 105 such that the calculated electrical power is equal or close to the target power.
FIG. 6 is an example of the current monitor 102 that includes a sensing resistor 112 and a differential amplifier 113 that amplifies a voltage difference caused by the sensing resistor 112.
The conventional heater controller 100 shown FIG. 6 inherently has a subject that the controller is necessary to provide, in addition to the voltage monitor to monitor the voltages of both ends of the heater H, the current monitor 102 to monitor the current, which forces the size of the controller in large. Recent optical module applicable for the tunable LD, for instance, a tunable XFP optical transceiver, requests the package thereof as small as possible. So the heater controller with the conventional current monitor is inconsistent with such request.